
As you have heard, tonsils swell up (as do other glands) in response to generalised infection, and tonsillitis is rarely an isolated illness. I would be concerned that something else is going on, so would recommend further veterinary investigation. It can be associated with pharyngitis, again usually a symptom of an infectious disease, or the swallowing of a foreign body. What breed is the bitch?
Hope you get to the bottom of this.